In recent years there has been an increasing demand for miniaturization of dielectric elements and improved reliability as electronic circuits reach higher densities. Electronic components such as laminated ceramic capacitors are rapidly becoming more compact, achieving higher capacity, and becoming more reliable while the applications of electronic components such as laminated ceramic capacitors are also expanding. As these applications expand, various electrical characteristics are required, such as temperature characteristics of capacitance in a laminated ceramic capacitor, capacitance during application of a DC electric field (DC bias), resistivity and reliability. In particular, in order to achieve miniaturization and increased capacity in a ceramic capacitor for medium- and high-voltage usage, which is employed at a high rated voltage (e.g., 100 V or more), improvements in dielectric constant during application of a DC bias, resistivity and reliability are required of dielectric compositions forming the dielectric layers of a ceramic capacitor for medium- and high-voltage usage.
In order to respond to the various requirements such as those mentioned above, various types of dielectric compositions comprising BaTiO3 (which has a high dielectric constant) as the main component have been investigated as dielectric compositions which are employed in laminated ceramic capacitors. Among these, it is known that dielectric compositions having a structure in which an auxiliary component is diffused in the surface region of BaTiO3 particles (what is known as a “core-shell” structure) can improve electrical characteristics such as the temperature characteristics of dielectric constant, by controlling the composition and diffusion range of the shell portion which constitutes the auxiliary component diffusion phase.
For example, the laminated ceramic capacitor described in Japanese Patent Application JP 2000-58377 A has a main component in a dielectric ceramic layer represented by the following compositional formula: {Ba1-xCaxO}mTiO2+αRe2O3+βMgO+γMnO (where Re2O3 is at least one selected from among Y2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3 and Yb2O3; α, β and γ express molar ratios such that 0.001≤α≤0.10, 0.001≤β≤0.12 and 0.001<γ≤0.12; and 1.000<m≤1.035 and 0.005<x≤0.22). The content of alkali metal oxide in the {Ba1-xCaxO}mTiO2 starting material used in the dielectric ceramic layer is no greater than 0.02 wt %.
The composition contains 0.2-5.0 parts by weight of either a first auxiliary component or a second auxiliary component with respect to 100 parts by weight of the main component. The first auxiliary component is an oxide of Li2O—(Si,Ti)O2-MO (where MO is at least one selected from Al2O3 and ZrO2). The second auxiliary component is an oxide of SiO2—TiO2—XO (where XO is at least one selected from among BaO, CaO, SrO, MgO, ZnO and MnO).
An internal electrode of the laminated ceramic capacitor described in Japanese Patent Application JP 2000-58377 A further comprises nickel or a nickel alloy.
The laminated ceramic capacitor described in Japanese Patent Application JP 2000-58377 A has a core-shell structure in which an auxiliary component is diffused in the region of the grain boundary, and has a core portion in which some of the BaTiO3 constituting the main component of the dielectric ceramic layer is substituted with CaTiO3, and as a result it is possible to restrict a reduction in the dielectric constant when a DC bias is applied and to adequately increase reliability and resistivity when a DC bias is applied.
However, when a high DC bias of 5 kV/mm or greater is applied, the proportion of the domain which undergoes polarization reversal (the region in which the direction of the polarization axis is arranged within the crystal particles) increases because the coercive electric field of the main-component BaTiO3 is low. That is to say, the direction of the polarization axis in the domain is likely to be arranged in the direction of application of DC bias. It is generally known that there is a reduction in dielectric constant as a result of the direction of the polarization axis in the domain being arranged in the direction of application of DC bias.
Furthermore, it is also assumed that laminated ceramic capacitors will be used in the future under higher ambient temperatures and higher voltages. Further improvements in the electrical characteristics would therefore be desirable.